Thin film interference filters are important components in systems for optical measurement and analysis, such as Raman spectroscopy and fluorescence microscopy. In particular, thin film interference filters, such as optical edge filters, notch filters, and/or laser line filters (LLF's), are advantageously used in such systems to block unwanted light that would otherwise constitute or generate spurious optical signals and swamp the signals to be detected and analyzed. Thus, failure or inadequate performance of these filters can be fatal to operation of a system in which they are utilized.
In general, interference filters are wavelength-selective by virtue of the interference effects that take place between incident and reflected waves at boundaries between materials having different refractive indices. This interference effect is exploited in interference filters, which typically include a dielectric stack composed of multiple alternating layers of two or more dielectric materials having different refractive indices. In the case of a filter which substantially reflects at least one band of wavelengths and substantially transmits at least a second band of wavelengths immediately adjacent to the first band, such that the filter enables separation of the two bands of wavelengths by redirecting the reflected band, the resulting filter is called a “dichroic beamsplitter,” or simply a “dichroic” filter.
In a typical interference filter, each of the respective layers of the filter stack is very thin, e.g., having an optical thickness (physical thickness times the refractive index of the layer) on the order of a quarter wavelength of light. These layers may be deposited on one or more substrates (e.g., a glass substrate) and in various configurations to provide one or more of long-wave-pass (also called long-pass), short-wave-pass (also called short-pass), band-pass, or band-rejection filter characteristics.
In the case of prior known edge filters, the filter is configured so as to exhibit a spectrum having a clearly defined edge, wherein unwanted light having wavelengths above or, alternatively, below a chosen “transition” wavelength λT is blocked, whereas desired light is transmitted on the opposite side of λT. Edge filters which transmit optical wavelengths longer than λT are called long-wave-pass (LWP) filters, and those that transmit wavelengths shorter than λT are short-wave-pass (SWP) filters.
FIGS. 1A and 1B schematically illustrate the spectral transmission of idealized LWP and SWP filters, respectively. As shown in FIG. 1A, an idealized LWP filter blocks light with wavelengths below λT, and transmits wavelengths above λT. Conversely, as shown in FIG. 1B, an idealized SWP filter transmits light with wavelengths below λT, and blocks light with wavelength above λT.
Edge steepness and the relative amount of transmitted light are important parameters in many filter applications. As shown in FIGS. 1A and 1B, an idealized edge filter has a precise wavelength edge represented by a vertical line at λT. As such, an idealized filter has an “edge steepness” (i.e. a change in wavelength over a defined range of transmission) of 0 at λT. However, real edge filters change from blocking to transmission over a small but non-zero range of wavelengths, with increasing values of edge steepness reflecting an edge that is increasingly less steep. The transition of a real edge filter is therefore more accurately represented by a non-vertical but steeply sloped line at or near λT. Similarly, while an ideal edge filter transmits all light in the transmission region (transmission T=1), real filters have some amount of transmission loss, invariably blocking a small portion of the light to be transmitted (T<1).
As a result, the reported edge steepness of a real edge filter depends on the transmission range over which it is defined. Further, as will be discussed below, conventional edge filters exhibit polarization splitting when operated at a non-zero angle of incidence, in which case the corresponding spectra for s and p-polarized light may not have the same edge steepness.
Edge filters, notch filters, and laser line filters are particularly useful in optical measurement and analysis systems that use light from a light source, such as a laser, to excite/illuminate a sample at one wavelength λL (or a small band of wavelengths) and measure or view an optical response of the excited sample at other wavelengths. The excitation light λL is delivered to the sample by an excitation light path, and the optical response of the sample is delivered to the eye or measuring instrument by a collection path. Notch filters are generally specialized implementations of edge filters, in that they exhibit a long wave edge and a short wave edge bordering a narrow region of low transmission. Laser line filters are generally configured so as to transmit as much light from a desired wavelength as possible, while blocking other wavelengths.
These filters have been used to block spurious or unwanted light from the excitation and collection paths of an optical system. In the case of edge filters, filters having higher edge steepness (i.e., a smaller difference in wavelength over a defined transmission range) are capable of more effectively blocking spurious or unwanted light signals. Further, edge filters having lower transmission loss, if placed in the collection path, are capable of passing more light from the sample to the measuring instrument. Similarly, LLF's having lower transmission loss, if placed in the excitation path, are capable of passing more excitation light from the light source (e.g., a laser) to the sample.
Raman spectroscopy is one example of an optical analysis system that advantageously employs dichroic/interference filters. In Raman spectroscopy, molecular material is irradiated with excitation light, i.e., high intensity light of a given wavelength λL (or range of wavelengths) Upon irradiation, the molecular material scatters the excitation light. A small portion of the scattered excitation light is “Raman shifted,” i.e., it is shifted in wavelength above and/or below λL. This Raman shifting is attributed to the interaction of the light with resonant molecular structures within the material, and the spectral distribution of the Raman shifted light provides a spectral “fingerprint” characteristic of the composition of the material. However, the bulk portion of the scattered excitation light is “Rayleigh scattered,” i.e., it is scattered without a shift in wavelength.
Because the amount of Raman shifted light is very small relative to the amount of Rayleigh scattered light, it is necessary to filter the Raleigh scattered light from the collection path before it reaches the detector. Without such filtering, the Rayleigh scattered light will swamp the detector, and may excite spurious Raman scattering in the collection path. Filtering of the Rayleigh scattered light can be accomplished, for example, by placing an edge filter, such as a LWP filter having a transition wavelength λT just above λL (or range of wavelengths) between the sample and the detector. In this position, the LWP filter ensures that the light reaching the detector is predominantly long-wavelength Raman-shifted light from the sample. Similar arrangements using edge filters can be used to analyze short wavelength Raman-shifted light.
In an ideal Raman spectroscopy setup, a filter, such as a notch or edge filter, is configured such that it blocks 100% of light having a wavelength λL (or range of wavelengths) from reaching the detector, while allowing desired light to be passed to the detector for measurement. This could be accomplished for example, if the filters were configured so as to exhibit an ideal stopband that blocks 100% of light having a wavelength λL (or range of wavelengths).
Conventional filters, however, exhibit narrow blocking or transmission bands that exhibit a level of transmission and/or blocking that is less than optimum. The “blocking” of a filter at a wavelength or over a region of wavelengths is typically measured in optical density (“OD” where OD=−log10(T), T being transmission of the filter at a particular wavelength). Conventional filters that achieve high OD values at certain wavelengths or wavelength regions may not necessarily also achieve high transmission (in excess of 50%, for example) at any other wavelengths or wavelength regions. High OD is generally exhibited in a fundamental “stopband” wavelength region, and such stopbands have associated with them higher-order harmonic stopband regions occurring at other wavelength regions.
These higher-order stopbands are one reason why it is difficult to achieve high transmission at wavelengths shorter than those over which high blocking occurs. A stopband is a range of wavelengths over which transmitted light is strongly attenuated (T≦10%) due to constructive interference of the many partial waves of light reflected off of a structure with a periodic or nearly periodic variation of the index of refraction, as found in a thin-film interference filter. For a “quarter wavelength stack” structure comprised of alternating layers of high- and low-index materials, each of which is approximately one quarter of a particular wavelength λ0 thick (in the material), the “fundamental” stopband is roughly centered on λ0 and ranges from approximately λ0/(1+x) to λ0/(1−x), where x is related to the high and low index of refraction values, nH and nL, respectively, according to
  x  =            2      π        ⁢                  arcsin        ⁡                  (                                                    n                H                            -                              n                L                                                                    n                H                            +                              n                L                                              )                    .      
If the layer-to-layer index of refraction variation is not a purely sinusoidal variation, but rather changes abruptly, as is typically the case in a multi-layer thin-film interference filter, higher-order stopbands exist at shorter wavelengths. For example, a quarter-wave stack having such abrupt refractive index changes exhibits “odd-harmonic” stopbands that occur approximately at the wavelengths λ0/3, λ0/5, etc., and that range from approximately λ0/(3+x) to λ0/(3−x), for the third-order stopband, λ0/(5+x) to λ0/(5−x), for the fifth-order stopband, and so on. If the layers are not exactly a quarter-wave thick, there may also be “even-harmonic” stopbands that occur approximately at the wavelengths λ0/2, λ0/4, etc.
In general, known filters achieve high blocking over a wide range by utilizing a fundamental stopband, by combining multiple fundamental stopbands, or by “chirping” (gradually varying) the layers associated with one or more fundamental stopbands. Regardless of the approach, the higher-order harmonic stopbands associated with these blocking layers inhibit transmission at wavelengths shorter than the fundamental stopband or stopbands.
FIG. 2 schematically illustrates a Raman spectroscopy system 10 having a standard configuration. As shown, this standard configuration includes a light source 1, such as a laser, an excitation filter 2, a sample 3, a collection filter 4, and a detector 5. In operation, light source 1 emits light having a wavelength λL (or range of wavelengths) which passes though excitation filter 2 and illuminates sample 3 directly. Sample 3 scatters Raman shifted and unshifted excitation (Rayleigh scattered) light. Collection filter 4 is positioned between sample 3 and detector 5, such as a spectrophotometer. Collection filter 4 is configured to block the Rayleigh scattered light from sample 3 but transmit as much of the Raman shifted light as possible, and as close to λL as possible.
In focusing or imaging systems that utilize high numerical aperture (high-NA) collection optics, however, it is desirable for light from the light source and the collected signal light to share a common path. To meet this requirement, a two-filter solution is ideal. FIG. 3 schematically illustrates a Raman spectroscopy system 20 having such a configuration.
As shown, this configuration generally includes a light source 11, such as a laser, an excitation filter 12, a sample 13, a collection filter 14, a detector 15, such as a spectrophotometer, and a beamsplitter optical filter 16. Beamsplitter optical filter 16 is oriented at non-zero angle of incidence, e.g., about 45°, relative to light incident from light source 11, and is configured to reflect incident light from light source 11 onto sample 13, while transmitting Raman scattered light returning from Sample 13. Collection filter 14 is used in conjunction with beamsplitter optical filter 16 to ensure complete blocking of incident light that is Rayleigh scattered or reflected from sample 13. Due to the orientation of beamsplitter optical filter 16 relative to light from light source 11, the system shown in FIG. 3 is configured for so called, “non-zero angle of incidence” spectroscopy.
Increasing the angle of incidence of a traditional interference filter from normal generally affects the spectrum of the filter in two ways. First, the features of the filter spectrum are shifted to shorter wavelengths. And second, as the angle of the filter is further increased from normal, the filter spectrum exhibits progressively increasing “polarization splitting.” That is, the filter produces two distinct spectra, one for s-polarized light, and one for p-polarized light. The relative difference between the s and p spectra at a given point is generally called “polarization splitting.”
To illustrate this principal, reference is made to FIGS. 4A and 4B which are plots of polarization splitting vs. angle of incidence for a quarter wave stack based on SiO2 and Ta2O5 centered at 500 nm. In the plot of FIG. 4A, the bandwidths of the stopbands associated with light of s polarization and p polarization are shown, with the bandwidths measured in so-called “g-space.” The parameter g=λ0/λ is inversely proportional to wavelength and therefore directly proportional to optical frequency, and equals 1 at the wavelength λ0 which is at the center of a fundamental stopband associated with a stack of thin film layers each equal to λ0/4 n in thickness, where n is the index of refraction of each layer. The bandwidth in g-space is therefore equal to the difference between λ0/λS and λ0/λL, where λS and λL are the short-wavelength and long-wavelength edges of the stopband, respectively. The polarization splitting in g-space is thus simply one half of the difference between the bandwidths in g-space for s-polarized and p-polarized light. As shown in FIG. 4B, the stack exhibits polarization splitting of about 0.04 g-numbers when operated at 45° AOI. Increasing AOI to 60° results in polarization splitting of almost 0.08 g-numbers. Decreasing AOI to 20° results in polarization splitting of less than 0.02 g-numbers.
Many uses for thin film interference filters are known. For example, U.S. Pat. No. 7,068,430, which is incorporated herein by reference, discusses the use of such filters in Fluorescence spectroscopy and other quantification techniques.
Dichroic optical filters have been proposed for use in optical systems employing a two filter design, such as the one shown in FIG. 3. However, as described above and shown in FIG. 4, traditional dichroic filters exhibit substantial polarization splitting, particularly when operated at about 45° Angle of incidence. This polarization splitting arises from the particular construction of a dichroic filter. As mentioned previously, traditional dichroic filters are generally made up of alternating thin material layers having differing refractive index. In addition to the refractive index of each layer being different than that of an adjacent layer, the effective refractive indices of each individual layer differ with respect to different polarizations of light. That is, the effective refractive index for a layer is different for p-polarized light than it is for s-polarized light. As a result, s-polarized and p-polarized light are shifted to different degrees upon passing through each layer in a dichroic filter. This difference in shift ultimately offsets the filter spectra corresponding to these differing polarizations, resulting in polarization splitting.
If a traditional dichroic filter is based on the first order stopband of an angle-matched quarter-wave stack, estimating the polarization splitting between the stopband bandwidths of the filter is relatively straightforward. That is, assuming the dichroic filter is made up of two materials having indices of nH and nL, respectively, at 45° angle of incidence, the effective indices can be calculated as follows:
                              n          L          S                =                                            n              L              2                        -                                          sin                2                            ⁡                              (                AOI                )                                                                        (        1        )                                          n          H          S                =                                            n              H              2                        -                                          sin                2                            ⁡                              (                AOI                )                                                                        (        2        )                                          n          L          P                =                              n            L            2                                                              n                L                2                            -                                                sin                  2                                ⁡                                  (                  AOI                  )                                                                                        (        3        )                                          n          H          P                =                              n            H            2                                                              n                H                2                            -                                                sin                  2                                ⁡                                  (                  AOI                  )                                                                                        (        4        )            Wherein:                AOI is the incident angle in air, which is assumed to the incident medium;        nLP and nLS are the effective refractive index of the low index material in the dichroic stack for p-polarized light and s-polarized light, respectively; \        nHP and nHS are the effective refractive index of the high index material in the dichroic stack for p-polarized light and s-polarized light, respectively; and        nH2 and nHS are the squares of the high and low refractive indexes, respectively, associated with the two materials, and which are independent of polarization.        
The bandwidths and polarization splitting of the first-order stopband for the two polarizations may then be calculated as follows:
                              Δ          ⁢                                          ⁢                      g            S                          =                              4            π                    ⁢                                    sin                              -                1                                      ⁡                          (                                                                    n                    H                    S                                    -                                      n                    L                    S                                                                                        n                    H                    S                                    +                                      n                    L                    S                                                              )                                                          (        5        )                                          Δ          ⁢                                          ⁢                      g            P                          =                              4            π                    ⁢                                    sin                              -                1                                      ⁡                          (                                                                    n                    H                    P                                    -                                      n                    L                    P                                                                                        n                    H                    P                                    +                                      n                    L                    P                                                              )                                                          (        6        )                                          PS          g                =                                            Δ              ⁢                                                          ⁢                              g                S                                      -                          Δ              ⁢                                                          ⁢                              g                P                                              2                                    (        7        )            Wherein:                ΔgS and ΔgP are the bandwidths of the first order (fundamental) stopband for s-polarized light and p-polarized light, respectively, in g-space; and        PSg is the polarization splitting for the first-order stopband in g-space.Alternatively, the polarization splitting may be expressed in terms of wavelength. For example,        
                              PS          λ                =                                            λ              0                                      1              -                              Δ                ⁢                                                                  ⁢                                                      g                    S                                    /                  2                                                              -                                    λ              0                                      1              -                              Δ                ⁢                                                                  ⁢                                                      g                    P                                    /                  2                                                                                        (        8        )            wherein:                PSλ is the polarization splitting of the long-wavelength edge of the fundamental stopband (the edge associated with a long-pass filter).Often this value is expressed as a dimensionless value by taking its ratio to the average wavelength of the edges associated with s- and p-polarizations and expressing it as a percentage.        
Polarization splitting has been utilized to design polarizing filters where high transmission and blocking are achieved for s and p polarizations, respectively, over a defined wavelength band. However, in the context of edge filters and beamsplitter optical filters, polarization splitting severely limits the edge steepness of light having average polarization. Thus, it is desirable to minimize polarization splitting as much as possible.
Several ways have been proposed to minimize polarization splitting. For example, one method proposed by Thelen (See A. Thelen, “Design of Optical Interference Coatings,” McGraw Hill, 1989) utilizes tuning spacers of a multi-cavity Fabry-Perot bandpass filter to align the edges of spectrum of s and p-polarized light. However, this method has significant limitations when used to create dichroic filters.
In Thelen's method, the starting layer structure is that of a multi-cavity Fabry-Perot bandpass filter with spacer layers having optical thickness equal to multiple half-waves of the reference wavelength used to define the associated stopband. In addition, the edge of the resulting dichroic must be essentially at the center of the associated stopband. This is unlike the filters according to the present disclosure discussed below, which differ from Thelen's approach both in layer structure and placement of the dichroic edge with respect to the stopband. Indeed, as discussed below, filters according to the present disclosure do not contain the spacer layers required by Thelen's approach, and the dichroic edge may be placed virtually anywhere with respect to the location of the stopband.
In addition, it has been shown that decreasing stopband bandwidth can result in a corresponding decrease in polarization splitting. In the case of a filter having a second order stopband, the bandwidth of the stopband is proportional to the material mismatch in the dielectric stack making up the filter, where “mismatch” refers to the deviation of the layer thicknesses from one quarter of a wavelength, while keeping the sum of the thicknesses of each pair of high- and low-index layers equal to approximately one half of a wavelength. The greater the mismatch, the higher the degree of polarization splitting, and vice versa. Thus, it has been shown that polarization splitting can be minimized by utilizing different (e.g., higher-order) stopbands and adjusting material mismatch in the dielectric stack making up a dichroic filter.
However, while this method is effective, small mismatch always results in a filter having a narrow blocking region and lower blocking level, which is often not acceptable. Enhancement of the blocking region can be achieved, but only by increasing the number of layers in the dielectric stack. As a result, the performance of a traditional dichroic filter based on a second order stopband is typically limited by the maximum coating thickness allowed by the manufacturing process.
In addition, dual notch dichroic beamsplitters have been proposed for use in optical systems having dual filter designs. FIG. 5 is a measured spectrum of unpolarized light passing through an exemplary dual notch dichroic beamsplitter. As shown, this filter exhibits two narrow stopband regions 62 and 64 separated by a passband region having very narrow bandwidth 66. The spectrum also exhibits a relatively narrow bandpass region 68 between stopband region 64 and a fundamental stopband above about 750 nm (not shown)
While prior known interference filters are useful for many applications, they generally exhibit unsatisfactory characteristics when operated at about 45° angle of incidence. For example, the dual notch filter shown in FIG. 5 exhibits polarization splitting of 0.58% at one edge of stopband 62, and 0.4% at one edge of stopband 64. However, this filter exhibits a relative passband bandwidth of only about 30%, which is unsatisfactory. The relative passband bandwidth is the ratio of the difference between the long-wavelength of the passband and the dichroic edge wavelength to the dichroic edge wavelength (for a long-pass dichroic filter). Further, this filter exhibits relatively poor edge steepness of 1.26% at one edge of stopband 62, and 0.79% at one edge of stopband 64, which are insufficient for many applications. The edge steepness here is defined as the normalized wavelength difference between 10% and 90% transmission wavelengths for average polarized light.
Finally, angle matched notch filters have also been proposed for use in non-zero angle of incidence spectroscopy. Notch filters are described in detail in U.S. Pat. No. 7,123,416, the contents of which are incorporated herein by reference. However, when these filters are operated at about 45° Angle of incidence, they suffer from significant polarization splitting, as shown in FIG. 6 (where 72, 74, and 76 correspond to the s spectrum, p spectrum, and average spectrum, respectively) F and described above. Accordingly, these types of filters exhibit significant limitations when used in many optical measurement techniques.
Thus, there is a need for improved interference filters that, when operated at about 45° angle of incidence, exhibit substantially improved properties relative to prior known filters. In particular, there is a desire in the art for improved interference filters that, when operated at about 45° angle of incidence, exhibit at least one of improved polarization splitting, passband bandwidth, edge steepness, and blocking, relative to prior known filters.